1. Field of the Invention
The present invention relates to a magnetic disk which is mainly used in a computer peripheral memory, a method of manufacturing a magnetic disk and a magnetic recording apparatus.
2. Description of the Related Art
A magnetic disk has features such as high recording density, high data transfer speed, high-speed access, high reliability and low price, and it is the mainstream of computer peripheral memory. The recording density of a magnetic disk has increased a dozen times over the past ten years, so it is expected to continue to be improved.
The principle of magnetic recording is such that a string of recording magnetic domain domains are formed in a magnetic recording layer by a signal magnetic field generated from a magnetic head so that recording is performed. The signal magnetic field which is leaked from a string of the recording magnetic domains to the outside of the recording layer is reproduced by the magnetic head. In order to improve the recording density, the recording magnetic domains to be formed in the magnetic recording layer are to be made minute as possible, and a microscopic magnetic field which is leaked from a minute recording magnetic domain is reproduced at as high a density as possible.
In order to make the recording magnetic domain minute, it is important that first, the point of the magnetic head which is closest to the magnetic recording layer is made minute, more specifically, a recording (reproducing) magnetic gap is made narrow and a width of a magnetic pole track is made narrow. Secondly, a space between the top of the magnetic head and the magnetic recording layer is made narrow. Thirdly, fringing of the fringe of the recording magnetic domain due to a magnetic field dispersing from the point of the magnetic head spatially is lowered as much as possible. Fourthly, the point of the magnetic head is located in a prescribed recording/reproducing position of the magnetic recording layer as accurately as possible.
In addition, a break-through in reproducing principles is required for reproducing a microscopic magnetic field from a minute recording magnetic domain at high density. In recent years, a reproducing principle utilizing a magnetoresistance effect which is different from a conventional induction reproducing principle has been suggested and demonstrated, and a material having a big magnetoresistance effect has been researched and developed. Therefore, this principle is considered to be a mainstream of reproduction of a microscopic magnetic field.
The above-mentioned technical points of higher recording/reproducing density are common to a head floating-type recording/reproducing method (induction reproducing-type) using a longitudinal medium applied to current magnetic recording and to a head contact-type recording/reproducing method (magnetoresistance reproducing type) using a perpendicular medium.
Conventionally, examples of means of reducing the fringing are a means for reducing a fringe magnetic field from a head by narrowing a space and a means for reducing a width of magnetization transition in a recording layer. However, since it is impossible to eliminate fringing in a conventional magnetic disk from a viewpoint of the principle, a track should have enough width in expectation of a certain fringe value. This hinders the narrowing of a track.
In addition, after a magnetic recording disk and a magnetic head are mounted on a drive, a magnetic servo signal and address signal are recorded in a recording layer by using a servo writer, and the head is located by using the servo information during the actual operation. However, the tracking accuracy is limited to mechanical accuracy of the head as long as the magnetic recording layer has a simple continuous plane. This also hinders the narrowing of a track.
Japanese Laid-Open Patent Application No. 2-201730 discloses techniques for achieving tracking with high accuracy. With this technique, physical unevenness is previously provided to a magnetic disk substrate and a magnetic recording layer is formed thereon, and a track is servo-controlled by utilizing a difference between a signal from a concave section and a signal from a convex portion. In such a PERM (Pre-Embossed Rigid Magnetic) disk, the tracking accuracy is determined according to accuracy of the physical unevenness of the substrate, and high accuracy such that fluctuation quantity is about 0.01 .mu.m order can be realized by providing convex and concave section according to a process of an optical disk substrate.
However, in the PERM disk, since a member composing a guard band is made of a soft resist, only the resist is liable to wear selectively. In order to prevent the wear, it is required to coat the surface of the disk with a protective film, and as a result, it is hard to narrow a space. Therefore, synthetically, this is not a high-accuracy technique.
Japanese Laid-Open Patent Application No. 2-189715 discloses a magnetic recording medium in which an organic thick film such as a resist is provided on a substrate, and physical unevenness is provided on the surface of the organic film by pressing an uneven stamper against the organic film, etc. A magnetic thin film is buried in the concave section, and the organic film and the magnetic film essentially form a plane. In this magnetic recording medium, since the organic film exists below the magnetic film, a thick interface invalid layer is formed below the magnetic film. As a result, in order to obtain a magnetic film having a prescribed characteristic, the thickness of the magnetic film should be thick, so the high-resolution recording is difficult. Therefore, this is not a high density technique. Moreover, in this conventional technique, since a high permeable film cannot be provided below the recording magnetic film, it is difficult to apply this technique to perpendicular magnetic recording which is expected to be a future high density recording technique.
In addition, Japanese Patent Application No. 5-205257 suggests a technique of implanting ions into an area between recording tracks of a magnetic recording layer, projecting a laser beam or the like thereto so as to eliminate a recording function and forming a guard band. However, in this conventional technique, since an affected zone of a recording layer is used as the guard band, it is difficult to form a uniform guard band. Moreover, there arises a problem that a boundary between the recording track and the guard band becomes unclear.
The "magnetoresistance effect" is a phenomenon such that an electric resistance value of a magnetic thin film used for reproduction is changed based on a relative angle between a direction of a current and a direction of magnetization of the reproducing magnetic substance. Compared with conventional induction reproduction, this effect has advantages such that reproducing sensitivity is very high and reproduced signal strength does not depend on a head running speed.
As magnetic substances to be used as a magnetoresistance head, four kinds of substances are provided:
an NiFe monolayer film utilizing anisotropy magnetoresistance effect; a CoFe/Cu/CoFe triple layer film having a spin valve structure in which an electrically conductive non-magnetic film is sandwiched between two magnetic thin films; NiFe/Ag having a granular structure in which magnetic grains are dispersed in an electrically conductive non-magnetic member; and a (Co/Cu)n film having a magnetic artificial multi-layer structure in which a lot of magnetic thin films and a lot of electrically conductive non-magnetic thin films are alternately laminated.
As to the film of a monolayer structure in the above substances, since elements can be formed comparatively in a simple manner, it has a practical use, but a change rate of resistance is 2% at most. Therefore, in the case where recording magnetic domains are made more minute, namely, a reproducing magnetic field is made minute, and the film of a monolayer structure is insufficient from a viewpoint of reproducing sensitivity.
In addition, the granular structure and the magnetic artificial multi-layer structure show a change rate of resistance of not less than dozens %, so they are expected to be used very much in the future. However, since a strong magnetic field of several kOe--dozens kOe is required for obtaining a great changing rate of resistance, the reproduction of a very weak medium magnetic field is difficult using this technique.
Therefore, the spin valve structure, which shows a small change in a magnetic field of less than several hundred Oe and a changing rate of resistance of about 10% becomes sufficiently practical, the center of public attention, and it is expected to be put to practical use as a next reproducing element in the monolayer film structure. The reproducing principle in the spin valve structure is that electric resistivity is changed according to a relative direction of magnetization in two magnetic thin films. While one magnetization direction of the magnetic thin film is fixed, the other magnetization direction of the magnetic thin film is aligned in the direction of a medium magnetic field so that a relationship of the relative magnetization direction between the two magnetic thin films is changed. In order to fix the magnetization of one magnetic thin film, a magnetic film with a high coercive force as a magnetization sticking film is exchange-coupled to the magnetic thin film so as to obtain a magnetization fixing film. Since the magnetization of the other magnetic thin film is rotated and aligned in that of a medium magnetic field, hereinafter, this film is referred to as "magnetization rotating film". In order to obtain symmetry of a reproduced signal, it is important that the direction of the magnetization of the magnetization rotating film intersect perpendicularly to the direction of the magnetization of the magnetization fixing film without a medium magnetic field. Therefore, in accordance with the relationship of the direction of the medium magnetic field, it is preferable that the magnetization direction of the magnetization fixing film is set so as to be arranged (1) in the same direction of the medium magnetic field, namely, the direction which is perpendicular to the medium surface, and (2) in a state of no medium magnetic field, the magnetization direction of one magnetization rotating film is arranged in a widthwise direction of the track of the medium.
Some methods have adopted the above technique of setting the magnetization directions of the magnetization fixing film and the magnetization rotating film. A magnetization sticking film is generally used for setting the magnetization direction of the magnetization fixing film, and two methods are suggested for setting the magnetization direction of the magnetization rotating film. First, a method utilizing a magnetic field generated from a sense current is suggested. Secondly, a method of exchange-coupling or magnetostatically coupling a hard film for applying a suitable bias magnetic field to a magnetization rotating film is suggested.
However, in the method using a sense current, a sense current value is defined by a bias magnetic field to be applied to the magnetization rotating film, and accordingly outputs cannot be made large by large currents. Also, since the bias magnetic field for the magnetization rotating film acts upon the magnetization fixing film so as to invert its magnetization direction, it is difficult to obtain operational reliability.
On the other hand, in the method using a hard film bias, a number of manufacturing processes is increased due to the complication of a film structure of the head. Accordingly, it is difficult to provide a head at a low price.
Explanations were given as to the importance of bias at the time of using a magnetoresistance effect element and concrete means by illustrating the spin valve structure. However, in the other structures, using a magnetoresistance effect film, in order to prevent distortion of the waveform, it is important that the rotation of the magnetization is made symmetric with respect to the direction of the medium magnetic field, in practical use. Therefore, in order to use the magnetoresistance effect film, the magnetic field is biased by any means.